The present invention relates to a sheet feed mechanism and a method of controlling the same in a light beam scanning apparatus, and more particularly to a sheet feed mechanism and a method of controlling the same for two-dimensionally scanning a sheet with a light beam, e.g., for applying stimulating light to a stimulable phosphor sheet with a radiation image recorded thereon to read the recorded image from the sheet, or for applying a light beam to a photographic photosensitive material in the form of a sheet film to record an image thereon.
There has recently been developed and widely used a radiation image recording and reproducing system for producing the radiation-transmitted image of an object using a stimulable phosphor material capable of emitting light upon exposure to stimulating rays. When a stimulable phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is subsequently exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation.
In the radiation image recording and reproducing system employing such a stimulable phosphor, the radiation image information of an object such as a human body is stored in a sheet having a layer of stimulable phosphor (hereinafter referred to as a "stimulable phosphor sheet" or simply a "sheet"), and then the stimulable phosphor sheet is scanned with stimulating rays to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected to produce an image signal that is electrically processed for generating image information which is recorded on a recording medium such as a photographic photosensitive material or displayed as a visible image on a CRT or the like.
The radiation image recording and reproducing system includes an image readout device for reading a radiation image from a stimulable phosphor sheet. More specifically, the stimulable phosphor sheet is two-dimensionally scanned by a laser beam, and light emitted from the sheet upon exposure to the laser beam is detected on a time-series basis by a light detector such as a photomultiplier which produces an image signal representative of the image information. The stimulable phosphor sheet is two-dimensionally scanned by the laser beam by deflecting the laser beam one-dimensionally over the sheet in a main scanning direction and simultaneously feeding the sheet mechanically with a feed mechanism such as an endless belt conveyor in a subscanning direction normal to the main scanning direction.
The image information thus obtained from the stimulable phosphor sheet is then fed to an image recording device. The image recording device applies a laser beam modulated by the image information to a recording medium such as a photographic photosensitive material to record the image thereon. The image recorded on the photographic photosensitive material is thereafter developed, and the photographic photosensitive material is stored in a suitable location for use in medical diagnosis as required.
The stimulable phosphor sheet fed by the belt conveyor in the image readout device must be positioned stably on the belt conveyor. If the stimulable phosphor sheet were displaced on the belt conveyor during the scanning thereof, the light beam applied to the sheet would be displaced out of a desired position. As a result, if the stimulable phosphor sheet displaced on the belt conveyor were continuously scanned by the laser beam, image information obtained from the sheet would be inaccurate. Stated otherwise, no accurate radiation image information could be produced from the stimulable phosphor sheet suffering from a positional error. When the imaged object is a patient, a diagnostic error would tend to result from such inaccurate image information.
One conventional solution has been to use a suction box for holding a stimulable phosphor sheet being scanned stably on the belt conveyor without unwanted displacement. The suction box is positioned in a central space in the endless belt conveyor, and has a plurality of suction holes. When the stimulable phosphor sheet is delivered onto the belt conveyor, a vacuum generator coupled to the suction box is actuated to develope a vacuum in the suction box to attract the sheet under suction through the suction holes, thereby positioning the sheet stably on the belt conveyor. Therefore, the stimulable phosphor sheet is conveyed with the belt conveyor as it is moved in the subscanning direction.
The suction box however makes the feed mechanism complex and large, and requires the vacuum generator to enable the suction box to attract the sheet and also a control system for controlling the vacuum generator. The sheet positioning and feeding means of this construction is considerably costly, and so is the radiation image recording and reproducing system.
Another problem is that a feed path must be provided for feeding the stimulable phosphor sheet onto the belt conveyor in a direction parallel thereto in order to atract the sheet effectively on the belt conveyor. The radiation image recording and reproducing system with such a feed path is necessarily large in size. Where the system is installed in a hospital, for example, the size of the system makes it difficult to effectively utilize the space of a room in which it is located.
In the image recording device, the photographic photosensitive material is scanned in a main scanning direction by the laser beam which is modulated by the image information and cyclically deflected. At same time, the photographic photosensitive material is gripped between a large-diameter drum coupled to a rotative drive source and a pair of rollers on the drum and is fed thereby in a subscanning direction substantially normal to the main scanning direction.
The rotative drive source, such as a motor, for rotating the drum may be subjected to a load variation which leads to a failure of desired subscanning of the photographic photosensitive material.
Heretofore, any variations in the load on the motor during the subscanning process have been minimized by support bases disposed in front of and behind the drum in the subscanning direction. Each of the support bases must be of a length at least equal to or greater than the length of the photographic photosensitive material in the subscanning direction. Therefore, the image recording device is considerably large in size.
The drum has a width larger than the width of the photographic photosensitive material so as to be capable of stably feeding the material. Since the motor is disposed on one side of the drum in the direction of the width thereof, the image recording device is also of a large extent in the main scanning direction. For scanning the photographic photosensitive material highly accurately, it is necessary to control the motor highly precisely in order to convey the photographic photosensitive material at a constant speed in an accurate direction. Inasmuch as a motor capable of being controlled highly precisely is expensive, the cost of the image reading device is high.
The roller pair on the drum is required to feed the photographic photosensitive material stably. The roller pair however presents an obstacle to the scanning of the photographic photosenitive material at its opposite ends in the subscanning direction, failing to meet the demand to surround the produced image with a black frame or edge.